This invention relates to the modulating of signals on carriers, which are transmitted and the signals intelligibly recovered, and more particularly, to the modulation of speech on a carrier and the intelligible recover of the speech by means of the Radio Frequency Hearing Effect.
The Radio Frequency (xe2x80x9cRFxe2x80x9d) Hearing Effect was first noticed during World War II as a subjective xe2x80x9cclickxe2x80x9d produced by a pulsed radar signal when the transmitted power is above a xe2x80x9cthresholdxe2x80x9d level. Below the threshold level, the click cannot be heard.
The discovery of the Radio Frequency Hearing Effect suggested that a pulsed RF carrier could be encoded with an amplitude modulated (xe2x80x9cAMxe2x80x9d) envelope. In one approach to pulsed carrier modulation, it was assumed that the xe2x80x9cclickxe2x80x9d of the pulsed carrier was similar to a data sample and could be used to synthesize both simple and complex tones such as speech. Although pulsed carrier modulation can induce a subjective sensation for simple tones, it severely distorts the complex waveforms of speech, as has been confirmed experimentally.
The presence of this kind of distortion has prevented the click process for the encoding of intelligible speech. An example is provided by AM sampled data modulation.
Upon demodulation the perceived speech signal has some of the envelope characteristics of an audio signal. Consequently a message can be recognized as speech when a listener is preadvised that speech has been sent. However, if the listener does not know the content of the message, the audio signal is unintelligible.
The attempt to use the click process to encode speech has been based on the assumption that if simple tones can be encoded, speech can be encoded as well, but this is not so.
A simple tone can contain several distortions and still be perceived as a tone whereas the same degree of distortion applied to speech renders it unintelligible.
In accomplishing the foregoing and related object the invention uses a. modulation process with a fully suppressed carrier and pre-processor filtering of the input to produce an encoded output. Where amplitude modulation (AM) is employed and the pre-processor filtering is of audio speech input, intelligible subjective sound is produced when the encoded signal is demodulated by means of the RF Hearing Effect. Suitable forms of carrier suppressed modulation include single sideband (SSB) and carrier suppressed amplitude modulation (CSAM), with both sidebands present.
The invention further provides for analysis of the RF hearing phenomena based on an RF to acoustic transducer model. Analysis of the model suggests a new modulation process which permits the RF Hearing Effect to be used following the transmission of encoded speech.
In accordance with one aspect of the invention the preprocessing of an input speech signal takes place with a filter that de-emphasizes the high frequency content of the input speech signal. The de-emphasis can provide a signal reduction of about 40 dB (decibels) per decade. Further processing of the speech signal then takes place by adding a bias level and taking a root of the predistorted waveform. The resultant signal is used to modulated an RF carrier in the AM fully suppressed carrier mode, with single or double sidebands.
The modulated RF signal is demodulated by an RF to acoustic demodulator that produces an intelligible acoustic replication of the original input speech.
The RF Hearing Effect is explained and analyzed as a thermal to acoustic demodulating process. Energy absorption in a medium, such as the head, causes mechanical expansion and contraction, and thus an acoustic signal.
When the expansion and contraction take place in the head of an animal, the acoustic signal is passed by conduction to the inner ear where it is further processed as if it were an acoustic signal from the outer ear.
The RF to Acoustic Demodulator thus has characteristics which permit the conversion of the RF energy input to an acoustic output.
Accordingly, it is an object of the invention to provide a novel technique for the intelligible encoding of signals. A related object is to provide for the intelligible encoding of speech.
Another object of the invention is to make use of the Radio Frequency (xe2x80x9cRFxe2x80x9d) Hearing Effect in the intelligible demodulation of encoded signals, including speech.
Still another object of the invention is to suitably encode a pulsed RF carrier with an amplitude modulated (xe2x80x9cAMxe2x80x9d) envelope such that the modulation will be intelligibly demodulated by means of the RF Hearing Effect. A related object is to permit a message to be identified and understood as speech when a listener does not know beforehand that the message is speech.
Other aspects of the invention will become apparent after considering several illustrative embodiments, taken in conjunction with the drawings.